Seed planting machines have typically been equipped with one of two different systems for containing the supply of seeds on the machine and metering them to openers that place the seeds below the surface of the ground. In one system a large bulk container for the seed supply has a number of meters attached directly to the bottom of the container, as is typically found on grain drills. Seeds discharging from the meters pass through flexible hoses which are essentially empty except for the slow trickle of seeds coming out of the meters. The principal advantage of the bulk supply system is ease of filling and a large capacity so as to reduce the number of times the operator must stop to refill the seed container.
This system sometimes suffers from irregular seed spacing as the seeds are disturbed from their intended metered spacing by impinging against corrugations or bellows in the hoses that are there to permit the hoses to expand and retract to accommodate changes in the distance between the container and the opener as the opener moves over uneven terrain relative to the container. During such up and down movement of the opener, the hoses can also develop bends or slight curves, which position the internal hose surfaces at locations causing them to be impinged by the metered seeds, thus preventing their straight, unhindered drop to the openers.
Some machines have replaced the flexible hoses with telescoping, straight tubes between meters on the underside of the seed container and the openers. However, such arrangements do not address the problems of poor metering and irregular seed spacing caused by constant changes in the opener as it moves up and down relative to the container, such motion having the effect of constantly changing the distance a particular seed must travel from the meter to the soil.
A second type of system is one in which a small individual container of seeds is attached to each individual opener. In this system the seed metering device is attached to or built directly into the opener itself. This type of system is typically found on machines generically called "planters" in the industry.
In this type of machine the seeds travel from each individual container into a pool of seeds at the meter and then drop through the opener into the soil. This system tends to have improved seed spacing as the distance the seed must travel after leaving the metering device is constant relative to the soil. The distance the seed must travel is also usually much smaller than on a typical grain drill, leaving less opportunity for the seed drop to be disrupted as the machine moves across the field.
However, the multiple small containers typically have a much reduced overall volume than a bulk seed container as found on a grain drill, necessitating much more frequent stops while seeding to replenish the seed supplies in the individual containers. The many small containers spread across the width of the entire machine each come with their own lids which must be removed and replaced, making filling less convenient than with a central bulk container having only a single lid. Moreover, the individual small containers must typically be removed and inverted to empty excess seeds from them when changing from one crop to another, and when removing metering disks associated with the meters during service or for other purposes.
Furthermore these container/meter/opener units are typically much wider than the openers on grain drills, which prevents planters from achieving row spacing as narrow as that possible with drills.